This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A199239 #8 Jul 22 2025 13:15:06 %S A199239 1,2,6,440,29227,1682457,2013526831 %N A199239 Number of n X n 0..2 arrays with values 0..2 introduced in row major order, the number of instances of each value within one of each other, and every element equal to zero or two horizontal or vertical neighbors. %C A199239 Diagonal of A199246 %e A199239 Some solutions for n=4 %e A199239 ..0..1..0..1....0..1..1..2....0..1..0..2....0..1..1..0....0..1..0..1 %e A199239 ..1..2..2..0....2..1..1..0....2..0..2..1....2..1..1..2....1..0..2..0 %e A199239 ..0..2..2..1....0..2..0..2....1..2..1..0....1..0..2..0....0..2..0..2 %e A199239 ..2..1..0..2....2..1..2..0....2..0..2..1....0..2..0..2....2..1..2..1 %K A199239 nonn %O A199239 1,2 %A A199239 _R. H. Hardin_ Nov 04 2011